The present invention relates generally to sensing and displaying aircraft operability conditions, and more particularly to an inexpensive system that can provide flight data in the event of a primary instrument failure in general aviation aircraft.
Failure of a navigation system in general aviation (GA) aircraft presents a potentially dangerous scenario for pilots operating under instrument meteorological conditions (IMC). In a typical instrument-equipped GA aircraft, the displays include an attitude indicator (AI) and heading indicator (HI) that are both powered by an on-board vacuum-driven system. In such systems, the AI provides the only direct indication of aircraft pitch and bank attitude, and serves as the focal point of the pilot's instrument scan while performing what is commonly referred to as “attitude instrument flying”. An electric turn coordinator (TC, also referred to as a turn-and-slip or needle-and-ball in older installations) acts as a supporting and backup instrument for yaw rate (i.e., rotation about a vertical axis), roll rate (i.e., rotation about a longitudinal axis), and the balance of gravitational and centrifugal accelerations.
Experience shows that GA aircraft vacuum systems can be expected to fail on average about every 500 hours of operations. Many GA aircraft are not equipped with a standby vacuum system or a fault indicator to alert the pilot that the vacuum system or related equipment has suffered a failure. To make matters worse, a typical vacuum pressure gauge, which does provide an indication of an on-board vacuum failure, is usually small and placed in an obscure area of the instrument panel, well outside a pilot's normal instrument scan pattern. When the vacuum system does fail, the AI very slowly (usually imperceptibly) drifts into a false state in both pitch and bank, while “drift” errors slowly accumulate on the HI. The slow nature of the failure of these instruments almost invariably goes unnoticed by the pilot until suspicions are aroused by other factors, such as altitude/airspeed variations, wind/engine noise, or the like. Quite often the aircraft is in an unusual attitude by this time, and the confused and disoriented pilot is then called upon to recover the aircraft to a safe flight condition. This recovery must be based on the pilot's ability to integrate the indications of numerous instruments, all with serious inherent operational limitations (i.e., lags, etc.), and several of which (i.e., the gyroscopic instruments), probably unknown to the pilot, are providing misleading information. This type of “partial-panel” instrument flying is an exceptionally difficult task that few pilots perform willingly or well, and several documented studies have indicated that a significant percentage of aircraft mishaps arise from this situation, with a large majority of them proving to be fatal.
In a situation involving the loss of the AI and HI, a pilot may still glean certain information (for example, direction of turn) from the TC. Usually this instrument contains a single rate gyro mechanically linked to an indicator, for example displaying an aircraft tail-on profile against a fixed horizon line or scribe marks to indicate aircraft yaw or roll rate (in the case of a TC), or a single needle to indicate yaw rate (in the case of a turn-and-slip indicator). There is typically also an integral mechanical inclinometer (i.e., a free-moving ball in a liquid-filled tube) to indicate whether the gravitational forces are balanced by the centrifugal forces as a measure of the “quality” of the turn.
The TC is generally adequate for controlling aircraft bank angle in level flight or in turns of moderate rate. It is also useful in determining the correct direction to roll the aircraft to return to wings-level flight after entering an unusual attitude, as long as the ball in the mechanical inclinometer is near the center and the aircraft has not already rolled past 180° in one direction. Nevertheless, the TC provides no clues to aircraft pitch attitude. In situations involving loss of the AI and HI, this vital flight parameter must be inferred from observing the airspeed indicator, altimeter, and/or the vertical speed indicator (VSI). While the airspeed indicator may be correlated with known aircraft performance characteristics and current power level to gain insight into whether the nose is above or below the horizon, such method is rather imprecise. Likewise, the VSI is useful in maintaining level flight but has inherent lags that make precise control difficult. For example, while it is taught that VSI reversals indicate near-level flight in highly dynamic situations, the inherent lags and the fact that the instrument may be pegged at its upper or lower limit at the time of the reversal, reduce this technique to educated guesswork. The lower degree of lag in the altimeter, coupled with a lower likelihood of it being pegged, make it less problematic than the VSI in this regard. Despite this, the altimeter is not a panacea, as a reversal in the altimeter merely indicates near level flight, giving no direct indication of a level aircraft attitude, as angle-of-attack (AOA) variations may result from the combination of aircraft design, weight, acceleration and airspeed. Rapidly changing flight conditions (for example, acceleration and airspeed), in combination with lags inherent in other instruments such as the VSI, typically result in the pilot “chasing” the instruments during a dynamic upset recovery. The difficulty of a pilot performing flawlessly under such complex conditions is exacerbated by compelling yet erroneous pitch and bank information being provided by the AI that is situated at or near the center of the pilot's field-of-view.
Even if a pilot were to recover from the scenario mentioned above such that straight-and-level flight has been reestablished, there remains the problem of directional control. As mentioned above, a vacuum failure will also disable the gyroscopic HI. This leaves only a conventional magnetic (i.e., wet) compass for heading control, where such compass is susceptible to many degrading phenomena including deviation, variation, magnetic dip, acceleration error, northerly turning error and oscillation error. Compounding these inaccuracy problems is that the compass is frequently located well outside the normal instrument scan pattern, such as on the instrument glare shield or high on the windscreen.
A human-factors analysis of this problem clearly shows that one of the critical factors mitigating against safe partial-panel instrument flight is the necessity to modify the familiar instrument scan pattern and to correlate multiple instrument indications in order to form a clear picture of the flight situation; that is, to gain and maintain situation awareness. A contributing factor is that, in these circumstances, many instruments are being used for other than their primary intended purpose, and that they are not intuitive or optimally designed for that application. Clearly one approach to improving this situation is to “fuse” all the required critical information for partial-panel instrument flight into a single, intuitive, optimized display.
Accordingly, what is needed is a navigation system that can integrate various flight conditions (especially aircraft position information) into a single display to reduce pilot workload. What is further needed is a display that is reliable, relatively inexpensive to manufacture and easy to operate.